Pokemon EV training: Is it more sufficient to put EVs into HP or a Defense stat?

Well Defense is so your Pokemon loses less HP, HP is just for more HP. Ex, let's say I EV Train a Blissey in Defense. If it takes a hit from Mach Punch (this isn't real, it's just an example), let's say it's HP is yellow. Now, let's say we EVed it in HP instead, it takes the Mach Punch hit, and the HP is now 0 =O!